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In small streams, flow and water-quality concentrations often change quickly in response to meteorological events. Hydrologists, field technicians, or locally hired stream observers involved in water-data collection are often unable to reach streams quickly enough to observe or measure these rapid changes. Therefore, in hydrologic studies designed to describe changes in water quality, a combination of manual and automated sampling methods have commonly been used manual methods when flow is relatively stable and automated methods when flow is rapidly changing. Automated sampling, which makes use of equipment programmed to collect samples in response to changes in stage and flow of a stream, has been shown to be an effective method of sampling to describe the rapid changes in water quality (Graczyk and others, 1993). Because of the high cost of automated sampling, however, especially for studies examining a large number of sites, alternative methods have been considered for collecting samples during rapidly changing stream conditions. One such method employs the siphon sampler (fig. 1). also referred to as the "single-stage sampler." Siphon samplers are inexpensive to build (about $25$50 per sampler), operate, and maintain, so they are cost effective to use at a large number of sites. Their ability to collect samples representing the average quality of water passing though the entire cross section of a stream, however, has not been fully demonstrated for many types of stream sites. The Inter-Agency Committee on Water Resources, Subcommittee on Sedimentation (ICWR-SS) developed and tested siphon samplers under laboratory conditions and concluded that siphon samplers are able to collect a sample representative of near-surface water quality during rising stages. ICWR-SS (1961) developed several models of samplers to collect representative samples for distinct ranges of stream velocity, water-surface surge, water temperature, and sediment size. The study concluded that siphon samplers are useful when sediment concentrations near the water surface are of value and sampling by other, possibly more accurate methods is not practical or feasible. Edwards and Glysson (1988) outlined some of the limitations of siphon samplers. The primary limitation was that, because samples are collected near the water surface at one point in the stream, adjustments may be needed to describe the vertical and horizontal distributions in water quality, especially if the stream transports large sand-size particles. This is also a limitation for automatic samplers because automatic samPlywood cover

Comparison of water-quality samples collected by siphon samplers and automatic samplers in Wisconsin